Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). A MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MR images may be created by applying currents to the gradient and RF coils according to known algorithms called “pulse sequences.” The selection of a pulse sequence determines the relative appearance of different tissue types in the resultant images. Various properties of tissue may be used to create images with a desirable contrast between different tissues.
One technique that has been developed to accelerate MR data acquisition is commonly referred to as “parallel imaging” or “partial parallel imaging”. Various parallel imaging methods exist, including Simultaneous Acquisition of Spatial Harmonics (SMASH), Automatic Simultaneous Acquisition of Spatial Harmonics (AUTO-SMASH), Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA), Parallel Magnetic Resonance Imaging with Adaptive Radius in k-space (PARS), Autocalibrating Reconstruction for Cartesian Sampling (ARC), and Anti-aliasing Partially Parallel Encoded Acquisition Reconstruction (APPEAR), among others. In parallel imaging, multiple receive coils acquire data from a region or volume of interest, where the data is undersampled, for example, in a phase-encoding direction so that only a fraction of k-space data is acquired in an image scan. Thus, parallel imaging is used to accelerate data acquisition in one or more dimensions by exploiting the spatial dependence of phased array coil sensitivity. Parallel imaging has not only been shown to be successful in reducing scan time, but also reducing image blurring and geometric distortions. Moreover, parallel imaging can be used to improve spatial or temporal resolution as well as provide increased volumetric coverage.
Parallel imaging may be used to increase the scan time efficiency of three dimensional (3D) MRI scans. However, 3D MRI scans are susceptible to motion artifacts because scan times are often long even after acceleration with parallel imaging and because any subject motion during the 3D scan potentially affects the entire volume measurement and image quality of the entire acquired volume. Prospective motion correction techniques can be used to reduce motion-related artifacts in MR images. Prospective motion correction methods are implemented during image acquisition. Such methods estimate motion in the volume of interest and adjust the scan coordinates for each acquired k-space data segment accordingly so that the scan coordinates realign with the target volume in the event of motion. The adjustment of scan coordinates to align with the moving subject, however, effectively results in a stationary object and moving coil condition (assuming a rigid coil), yielding different coil sensitivity weightings for the data acquired before and after realignment. Because the unaliasing coefficients in parallel imaging depend directly or indirectly on coil sensitivity, employment of conventional parallel imaging techniques to prospective motion corrected data leads to erroneous image reconstruction, manifested in the resultant images as aliasing and other artifacts.
It would be desirable to have a system and method for generating MR images using both prospective motion correction and parallel imaging.